Q: re 555 timer monostable circuit that doesn't want to timeout. (solved)

[wilfred]

/.

[Zero999]

Does the relay have a protection diode in reverse parallel with it and another diode in series with it?

[dcel]

You are going to need a transistor or a MOSFET to run the relay. Don't forget the anti-kickback diode. You should have no problem finding the schematics online. A scan of your penciled schematic is very acceptable in order to help you.

Good luck

Chris

[dr_p]

22-680pF across the relay?


edit: but firstly, what happens to your power rail when the relay switches? Put a scope on it.

[dcel]

Looks to me that the additional circuit on pg2 is there for debounce. It also may have the effect of lowering the impedance of the input, thus lowering the sensitivity. Someone smarter than me will have to confirm that regarding the 555, I haven't played or built a project with a 555 since around 1990, when I built a dome light minder for my car.

Chris

[oldway]

1N4004 is far too slow, you should use a fast free wheeling diode in parallel with the relay.
1N4148 of UF4004 for example.

[Zero999]

You may find adding another diode in series with the relay might help.

[Andy Watson]

Pin 2 is a relatively high-impedance input. When you open the trigger switch the pin will float, probably to a voltage slightly above its trigger level, and due to its high impedance it will be susceptible to noise.  By putting the 100k resistor to Vcc you've stopped it floating and pulled the voltage away from its trigger point to its inactive state. When you've worked out why that has the effect that it does you might want to look at pin 4 - that also appears to float when the switch is open. As to why it works with an LED and not with a relay, it's probably due to the extra noise introduced by the relay - and luck (or un-luck!)